Trichinella spiralis is an intracellular nematode parasite of mammalian skeletal muscle cells that causes severe myositis and, sometimes, death in humans. The muscle infection lasts months to years, and domestic and wild mammals represent a continual environmental reservoir for human infection. When infected, terminally differentiated skeletal muscle cells are induced to re-enter the cell cycle, replicate DNA, then become terminally suspended in G2/M and cease muscle gene expression. The nuclei of these cells also become abnormally large (14-17 mu m), suggesting high transcriptional activity. These changes are thought to promote persistence of the intracellular parasite. We have identified parasite antigens that localize to host nuclei and likely regulate some of these changes. Methods were developed to extract these nuclear antigens (NA). It was further shown that mebendazole treatment of mice depletes NA from host nuclei and simultaneously leads to diminution in size of infected cell nuclei and decreased protein, RNA and acid phosphatase levels in infected cells, suggesting a causal relationship. Hence, methods are available to investigate parasite products that we hypothesize to play a central role in regulating the infected cell phenotype. Specific aims to investigate the functions of NA in the host cell are as follows: 1. Clone and evaluate NA genes from T. spiralis by screening a parasite cDNA expression library with antibodies made against NA from infected cell nuclei. 2. Determine the effects of NA gene expression on differentiation and reversal of differentiation in mammalian skeletal muscle cells. 3. Use the yeast two-hybrid system to identify host proteins that interact with NA proteins. Results from the described research are expected to clarify important molecular interactions that regulate this host/parasite relationship. The parasite undermines basic, host cell-cycle and gene regulatory pathways by mechanisms which might provide targets for improved treatments of trichinellosis. Understanding these mechanisms may also contribute unique insight on mechanisms of muscle differentiation and cell cycle regulation in mammalian cells.